1. Art and Diseño

Global warming slowdown: No systematic
errors in climate models
2 February 2015
analysis. They also clearly showed that the models
do not generally overestimate man-made climate
change. Global warming is therefore highly likely to
reach critical proportions by the end of the century if the global community does not finally get to grips
with the problem.
Forecasts without systematic errors: climate models,
such as the model MPI - ESM LR of the Max Planck
Institute for Meteorology, predict a significant increase in
temperature by the end of this century, especially at the
Earth's poles. No model, however, has predicted the
global warming hiatus which climate researchers have
observed since the turn of the millennium. This,
however, is not due to systematic errors of the models,
but to random fluctuations in the climate system. The
model predictions are therefore reliable, taking some
statistical uncertainty into account. Credit: MPI for
Meteorology / Deutsches Klimarechenzentrum (DKRZ)
Sceptics who still doubt anthropogenic climate
change have now been stripped of one of their lastditch arguments: It is true that there has been a
warming hiatus and that the surface of the earth
has warmed up much less rapidly since the turn of
the millennium than all the relevant climate models
had predicted. However, the gap between the
calculated and measured warming is not due to
systematic errors of the models, as the sceptics
had suspected, but because there are always
random fluctuations in the Earth's climate.
Recently, Jochem Marotzke, Director at the Max
Planck Institute for Meteorology in Hamburg, and
Piers M. Forster, a professor at the University of
Leeds in the UK, have impressively demonstrated
this by means of a comprehensive statistical
Climate is subject to chance and chaos - which
makes life difficult for climate researchers. No
wonder that these two unpredictable climate factors
lie at the root of a mystery that has baffled
scientists since the start of the 21st century. Since
then, the temperature of the Earth's surface has
increased by only around 0.06 degrees Celsius much less than had been predicted by all 114
model simulations considered in the climate report
by the IPCC. Jochem Marotzke and Piers M.
Forster have now explained the warming pause in
terms of random fluctuations arising from chaotic
processes in the climate system. Even more
importantly for the two researchers and their
colleagues around the world: they did not find any
conceptual errors in the models. Most notably, the
models do not generally react too sensitively to
increases in atmospheric carbon dioxide.
"The claim that climate models systematically
overestimate global warming caused by rising
greenhouse gas concentrations is wrong," says
Jochem Marotzke. Climate sceptics often make
precisely this claim, citing the warming pause as
evidence. Yet they cannot deny that nine of the ten
warmest years since systematic climate
observations began have occurred in the new
millennium and that global warming has slowed at a
very high level. The sceptics also ignore the fact
that ocean temperatures continue to rise as rapidly
as many models have predicted.
"On the whole, the simulated trends agreed well
with the observations"
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To explain the puzzling discrepancy between model
simulations and observations, Jochem Marotzke
and Piers M. Forster proceeded in two steps. First,
they compared simulated and observed
temperature trends over all 15-year periods since
the start of the 20th century. For each year
between 1900 and 2012 they considered the
temperature trend that each of the 114 available
models predicted for the subsequent 15 years.
They then compared the results with
measurements of how the temperature actually
rose or fell. By simulating the average global
temperature and other climatic variables of the past
and comparing the results with observations,
climatologists are able to check the reliability of
their models. If the simulations prove more or less
accurate in this respect, they can also provide
useful predictions for the future.
This image shows retroactively simulated and observed
15-year trends of the global mean surface temperature
since 1900. For each year from 1900 to 1998, the
15-year trend indicates how the temperature will change
over the next 15 years. Between 1900 and 1914, it
decreased for example, by about 0.09 degrees Celsius.
For this first year, the models predict a weaker negative
or even positive temperature trend ahead. The colour
shading indicates - based on the available 114
simulations - the frequency with which a simulated
temperature trend occurs for each start year. The circles
represent the observed temperature trends. For 1998,
the observed value is at the lower limit of the ensemble of
the simulations. This means that between 1998-2012, the
temperatures of the Earth's surface increased less
significantly on average than predicted by climate
models. Credit: Nature 2015/MPI for Meteorology
The 114 model calculations withstood the
comparison. Particularly as an ensemble, they
reflect reality quite well: "On the whole, the
simulated trends agree with the observations," says
Jochem Marotzke. The most pessimistic and most
optimistic predictions of warming in the 15
subsequent years for each given year usually
differed by around 0.3 degrees Celsius. However,
the majority of the models predicted a temperature
rise roughly midway between the two extremes.
The observed trends are sometimes at the upper
limit, sometimes at the lower limit, and often in the
middle, so that, taken together, the simulations
appear plausible. "In particular, the observed trends
are not skewed in any discernible way compared to
the simulations," Marotzke explains. If that were the
case, it would suggest a systematic error in the
models.
No physical reason explains the spread of the
predictions
In a second step, the two scientists are now
analysing why the simulations arrived at disparate
results. This analysis can also explain why the
various predictions for the past 15 years deviate
from the actual observed trend. Random
fluctuations and three physical reasons come into
question to explain this: The model calculations are
based on different amounts of radiant energy from
the sun that impinge on the Earth's surface and are
stored as a result of the greenhouse effect, e.g. due
to atmospheric carbon dioxide. However, their
predictions also respond with different degrees of
sensitivity to changes in this radiant energy, for
example if the carbon dioxide content of the
atmosphere doubles. In other words, the models
assume different proportions of energy that warm
the Earth's surface and the proportion that is
sooner or later radiated back into space. Finally, all
the climate models assume different amounts of
energy stored on the Earth that is transferred to the
ocean depths, which act as an enormous heat sink.
Using a statistical method, Marotzke and Forster
analysed the contributions of the individual factors
and found that none of the physical reasons
explains the distribution of predictions and the
deviation from the measurements. However,
random variation did explain these discrepancies
very well. In particular, the authors' analysis refutes
the claim that the models react too sensitively to
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increases in atmospheric carbon dioxide: "If
excessive sensitivity of the models caused the
models to calculate too great a temperature trend
over the past 15 years, the models that assume a
high sensitivity would calculate a greater
temperature trend than the others," Piers Forster
explains. But that is not the case, despite the fact
that some models are based on a degree of
sensitivity three times greater than others.
The Earth will continue to warm up
"The difference in sensitivity explains nothing
really," says Jochem Marotzke. "I only believed that
after I had very carefully scrutinised the data on
which our graphs are based." Until now, even
climatologists have assumed that their models
simulate different temperature rises because they
respond with different degrees of sensitivity to
increased amounts of solar energy in the
atmosphere. The community of climatologists will
greet this finding with relief, but perhaps also with
some disappointment. It is now clear that it is not
possible to make model predictions more accurate
by tweaking them - randomness does not respond
to tweaking.
Quite apart from their role as scientists,
researchers have another reason for greeting the
study with mixed feelings: no all-clear signal has
been sounded. Climatologists have been fairly
correct with their predictions. This means: if we
continue as before, the Earth will continue to warm
up - with consequences, particularly for developing
countries, that we can only begin to fathom.
More information: Jochem Marotzke & Piers M.
Forster, Forcing, feedback and internal variability in
global temperature trends, Nature, 29 January
2015; DOI: 10.1038/nature14117
Provided by Max Planck Society
APA citation: Global warming slowdown: No systematic errors in climate models (2015, February 2)
retrieved 6 February 2015 from http://phys.org/news/2015-02-global-slowdown-systematic-errors-
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